Electronic brake management system with manual fail safe

ABSTRACT

An improved electro-hydraulic brake system having features for improving the pedal feel of the system, while further having design features which contribute to the economy of manufacture of certain components of the system. The system provides for an electrically powered normal source of pressurized hydraulic brake fluid, and a manually powered backup source of pressurized hydraulic brake fluid to the vehicle brakes in the event of failure of the normal source. During normal braking, fluid from the backup source is redirected from the vehicle brakes to a pedal simulator. The pedal simulator preferably includes arrangements of spring loaded pistons, expansion volumes, and damping orifices, together with valves selectively controlling the flow of fluid to and from the pedal simulator which provides for an improved pedal feel during vehicle braking. The brake system of the invention further includes a relatively low cost fluid separator unit which is provided which prevents intermixing of pressurized fluid between the backup source and the normal source. The fluid separator unit acts to permit the normal source to act upon the hydraulic brake fluid of the backup source to operate the vehicle brakes. The fluid separator unit is preferably embodied as a piston having two working faces, each of the same diameter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/038,043 filed Mar. 6, 1997, U.S. Provisional Application No.60/032,595 filed Dec. 2, 1996, U.S. Provisional Application No.60/018,814 filed May 31, 1996, and U.S. Provisional Application No.60/013,005 filed Mar. 7, 1996, the disclosures of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to brake systems for ground vehicles,and in particular to electro-hydraulic brake systems with normal brakingpressure supplied by an electrically driven pump.

Electro-hydraulic braking systems with manually powered backup systemshave been shown in some publications. For example, German PatentApplication DE 4413579A1 illustrates a system having a manually poweredmaster cylinder connected through isolation valves to brakes at avehicle's wheels. When the isolation valves are shut, pressurized brakefluid from the master cylinder is delivered to a pedal simulator.Pressure transducers are used to develop a signal representative of adesired braking effort, which is fed to an electronic control unit. Theelectronic control unit controls the operation of motor operated brakingpressure generators (pumps) to correspondingly deliver pressurizedhydraulic brake fluid to the vehicle brakes.

SUMMARY OF THE INVENTION

While certain general principles of electro-hydraulic braking are known,the known equipment has been relatively expensive, and has hadrelatively poor functionality in such important areas as “pedal feel”,the tactile feedback a driver feels when operating the brake pedal ofsuch a brake system.

This invention relates to an improved electro-hydraulic brake systemhaving features for improving the pedal feel of the system, whilefurther having design features which contribute to an economy ofmanufacture of certain components of the system. The system provides foran electrically powered normal source of pressurized hydraulic brakefluid, and a manually powered backup source of pressurized hydraulicbrake fluid to the vehicle brakes in the event of failure of the normalsource. During normal braking, fluid from the backup source isredirected from the vehicle brakes to a pedal simulator. The pedalsimulator preferably includes arrangements of spring loaded pistons,expansion volumes, and damping orifices, together with valvesselectively controlling the flow of fluid to and from the pedalsimulator which provide for an improved pedal feel during vehiclebraking. The brake system of the invention further includes a relativelylow cost fluid separator unit which is provided to prevent intermixingof pressurized fluid between the backup source and the normal source.The fluid separator unit acts to permit the normal source to act uponthe hydraulic brake fluid of the backup source to operate the vehiclebrakes. The fluid separator unit is preferably embodied as a pistonhaving two working faces, each of the same diameter.

Various objects and advantages of this invention will become apparent tothose skilled in the art from the following detailed description of thepreferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of a vehicle brakesystem.

FIG. 2 is partial schematic view of the brake system of FIG. 1.

FIG. 3 is a graph of Pedal Force vs. Pedal Travel for anelectro-hydraulic brake system having an expansion volume unit and foran electro-hydraulic brake system not having an expansion volume unit.

FIG. 4 is a schematic view of a second embodiment of a vehicle brakesystem

FIG. 5 is a schematic view of a third embodiment of a vehicle brakesystem.

FIG. 6 is a sectional view of specific embodiments for a master cylinderand a pedal simulator which can be used for the brake systems of thepresent invention.

FIG. 7 is an enlarged cross-sectional view of a fluid separator pistonwhich may be used in the fluid separator units of FIG. 1.

FIG. 8 is an end elevational view of a piston which may be used in thefluid separator units of FIG. 1, showing a groove formed in the workingface thereof.

FIG. 9 is a view taken along the line 9-9 of FIG. 8.

FIG. 10 is a schematic view of a vehicle brake system having anelectro-hydraulic normal source and a backup source of pressurizedhydraulic brake fluid for the front brakes and individual power cylinderfor supplying pressurized hydraulic brake fluid to a respective rearwheel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There is shown in FIG. 1, a first embodiment of a vehicle brake system,indicated generally at 2, in accordance with the invention. The brakesystem 2 may suitably be used on a ground vehicle such as an automotivevehicle having four wheels and a brake for each wheel. The brake system2 includes a normal source of pressurized hydraulic brake fluid,indicated at 4, and a backup source of pressurized hydraulic brakefluid, indicated at 6. The normal source 4 includes an electroniccontrol module 10. The control module 10, as will be discussed below,receives various signals, processes these signals, and controls theoperation of various components of the brake system 2 based on thesesignals. In this manner, the control module 10 causes the normal source4 to cooperate with a portion of the hydraulic circuitry of the backupsource 6 to provide hydraulic brake fluid at electronically controlledpressures to four vehicle brakes 11a, b, c, and d. The vehicle brakes11a, b, c, and d each include a respective brake actuation member (suchas a slave cylinder) and friction member actuatable by the actuationmember for engaging a rotatable braking surface of the vehicle wheel.

The backup source 6 provides for manual backup braking for, preferably,two of the vehicle brakes 11a and 11b, as will be discussed in detailbelow. Generally, since the forward or front brakes of a vehicle providemost of the braking resistance in an automotive vehicle in the majorityof braking situations, it is envisioned that the front brakes will beconnected to the backup supply of pressurized hydraulic brake fluid.However, this invention could be easily adapted to function with anycombination of brakes, and is not limited to the configuration shown.

The source of pressurized hydraulic brake fluid for the backup source 6is a manually operated master cylinder 12. The master cylinder 12 isoperated by a brake pedal 14 to supply pressurized hydraulic brake fluidto a first manual backup brake circuit via a conduit 16 and a secondmanual backup brake circuit via a conduit 17. As shown, the mastercylinder 12 is preferably a tandem master cylinder, having two servicepistons, but the master cylinder 12 may be of any suitable design, suchas a single piston or triple piston design. The brake pedal 14 may beprovided with a brake pedal detector 18 to detect the movement of thebrake pedal 14. The brake pedal detector 18 may be a switch whichactuates the brake lights (not shown), or acts as an input to a controlmodule 10 to indicate that the brake pedal 14 is depressed. The brakepedal 14 is also preferably coupled to a displacement transducer 19producing a signal indicative of how far the brake pedal 14 isdepressed, which is indicative of brake demand by the operator, whichsignal can be an input to the control module 10. As is common, areservoir 20 is provided which communicates with the first and secondbrake circuits through the master cylinder 12 in the ordinary manner.The reservoir 20 may be a single, dual or triple chamber design, asappropriate, and indeed may have any suitable number of chambers.

The conduit 16 is connected via a first electrically operated isolationvalve 22a with a first hydraulically operated vehicle brake 11a. Theconduit 17 is connected via a second electrically operated isolationvalve 22b with a second hydraulically operated vehicle brake 11b. Whenan isolation valve 22a or 22b is electrically de-energized, the valve isopen, as shown in FIG. 1, allowing pressurized brake fluid from themaster cylinder 12 to be applied to the associated vehicle brake 11a or11b to brake the vehicle. In normal operation, the isolation valves 22aand 22b are deenergized open when no braking is occurring. The isolationvalves 22a and 22b are energized shut during vehicle braking, isolatingthe master cylinder 12 from the vehicle brakes 11a and 11b. In thiscondition, the pressurized brake fluid developed in the master cylinder12 is routed instead to a pedal simulator 26 via a conduit 27. Locatedin the conduit 27 is a simulator valve 28 for selectively allowing thepassage of fluid flowing into and out of the pedal simulator 26. Whenthe isolation valves 22a and 22b are energized shut, the simulator valve28 is energized open. When the isolation valves 22a and 22b aredeenergized open, the simulator valve 28 is deenergized shut. Theisolation valves 22a and 22b and the simulator valve 28 may be pulsewidth modulated to electronically command the operation of the valves.

As shown in detail in FIG. 6, the pedal simulator 26 includes a housing26a having a bore 26b. A piston 26c is slideably disposed within thebore 26b. The piston 26c is coupled to a cupped flanged member 26d.Rightward movement of the piston 26c from the position illustrated inFIG. 6 compresses a conical spring 26e against a plate 26f. The pedalsimulator 26 also includes an adjustable stop member 26g threaded intothe plate 26f which restricts the travel of the piston 26c and theflanged member 26d. The plate 26f is held in place against the force ofthe spring 26e by a snap ring 26h engaging a groove formed in thehousing 26a. An elongate member 26i couples the piston 26c and thecupped flanged member 26d to keep the piston 26c and the cupped flangedmember 26d in alignment and to transfer forces therebetween.

The pedal simulator 26 is connected to the conduit 16 so that when thebrake pedal 14 is depressed, pressurized brake fluid from the mastercylinder 12 is directed through the conduit 16 to the pedal simulator 26to drive the piston 26c to compress the spring 26e. As the spring 26ecompresses, the spring 26e exerts increased resistance to furthermovement of the piston 26c. As will be explained in detail below, thespring 26e preferably has a progressive rate, resulting in a greaterresistance to further movement, per unit of displacement of the brakepedal 14, when the brake pedal 14 is near the end of the pedal strokethan when the brake pedal 14 is first depressed. In this manner, thepedal simulator 26 can mimic the progressively greater incrementalresistance to pedal movement felt in conventional braking systems. Oneway of causing the spring 26e to have a progressive spring rate is toform the spring 26e as a conical helical spring with a varying pitch,that is, with each wrap of the spring 26e being inclined differentlyrelative to a plane (not shown) defined perpendicular to a central axis26j of the spring 26e.

As the spring 26e of the pedal simulator 26 exerts greater resistance,pressure in the conduit 16 is increased due to the resistance to furthermovement by the spring loaded piston 26c. This resistance to movement isfed back to the pedal 14 through the increased pressure of the conduit16 reacting in the master cylinder 12, so that the operator of the brakepedal feels an increasing resistance as the brake pedal 14 is depressed,similar to the resistance felt when the master cylinder 12 ishydraulically coupled to the vehicle brakes 11a and 11b. The pressure inthe conduit 17 will rise along with the pressure in the conduit 16 inthe ordinary manner For example, if the master cylinder 12 is a tandemaxial master cylinder, increased pressure in the primary chamber (notshown) of the master cylinder 12 and the conduit 16 is fed to thesecondary chamber (not shown) of the master cylinder 12 and the conduit17 by movement of the master cylinder secondary piston (not shown).

While the pedal simulator 26 preferably is embodied as the piston 26cacting against a single metal coil spring 26e, as shown in FIGS. 1 and6, other designs of pedal simulators are contemplated for use as part ofthe invention. For example, the pressurized hydraulic brake fluid in thepedal simulator 26 may act against any suitable spring arrangement suchas a plurality of coiled springs arranged to act in series or parallelto each other, and may suitably interact to deliver the desiredprogressive spring rate. Furthermore, the spring of the pedal simulator26 may be made of any suitable material. For example, the spring may bean elastomeric spring

The piston 26c of the pedal simulator 26 may be replaced by a diaphragmacting against a spring, or some other flexible or movable fluidseparator. As a further example, the pedal simulator 26 could include apiston, diaphragm, or bladder as a fluid separator, a first side ofwhich is acted upon by the pressurized brake fluid from the mastercylinder 12, and a second side of which is acted upon by a fluid, thepressure of which may increase naturally as the pressure in the brakecircuits increase (such as a fixed volume of gas), or which may beselectively controlled. It is specifically contemplated that thepressure of the fluid on the second side of such a fluid separator inthe pedal simulator 26 could be controlled to selectively adjust thedamping and spring rate characteristics of the pedal simulator 26. Suchpressure control could be achieved by any desired means, such aspressure feedback, electronic control of suitable pumps or valves orother mechanical actuators, or actuators achieving displacementprincipally due to a material therein undergoing a phase change.

It is also contemplated that such a fluid separator in the pedalsimulator 26 could be acted on directly by a selectively operatedmechanical actuator By controlling the spring rate and dampingcharacteristics of the pedal simulator 26, the pedal feel experienced bythe operator of the vehicle can be controlled when the brake pedal 14 isdepressed and released. In yet another design variation, the pedalsimulator 26 could be embodied as a chamber in which is situated anamount of a suitable material, such as a block of an elastomericmaterial, having a desired set of physical characteristics. The materialis elastically compressed as the pressure of the brake fluid in thepedal simulator 26 increases. The material could contain internalchambers filled with a gas.

The brake system 2 preferably includes an optional dampening circuit,shown schematically as block 29 in FIG. 1, and an optional expansionvolume unit, shown schematically as block 31 in FIG. 1. As will bediscussed in detail below, the dampening circuit 29 and the expansionvolume unit 31 cooperate with the pedal simulator 26 to provide forimproved brake pedal feel, which as indicated above, is the responsecharacteristic experienced by the operator of the vehicle whileoperating the brake pedal 14.

The pressure in the conduit 16 between the master cylinder 12 and theisolation valve 22a is monitored by a pressure transducer 30 whichsupplies a signal representative of the sensed pressure to the controlmodule 10 as a brake demand signal. Note that the signal from the brakepedal displacement transducer 19 may be used instead of the pressuresignal from the pressure transducer 30 as the brake demand signal, ormay be used as a backup or check signal to verify proper operation ofthe pressure transducer 30. If desired, the pressure in the conduit 17can also be monitored by a pressure transducer (not shown).

Preferably, however, the displacement signal from the pedal transducer19 and the pressure signal from the pressure transducer 30 are blendedtogether in a suitable fashion to create a system brake demand signal.For example, during the first portion of pedal travel, pressure measuredby the pressure transducer 30 does not increase greatly compared to theamount of pedal travel. It may be difficult to accurately determine thedesired braking demand from the pressure signal produced by the pressuretransducer 30, as the increase in the pressure signal may be difficultto differentiate from normal electronic background “noise”. Thus, in thefirst part of pedal travel, the signal from the pedal transducer 19 canbe a better indicator of desired braking, and can be given increasedweight in determining the brake demand signal. However, in the latterpart of the pedal stroke, the pressure monitored by the pressuretransducer 30 can change significantly with only a small change inposition of the brake pedal 14, and thus a relatively small change inthe brake pedal signal produced by the pedal transducer 19. Thus, inthis region, the signal from the pressure transducer 30 may be a moreaccurate determinator of the desired braking, and thus given greaterweight in determining the brake demand signal. In an intermediateportion of the pedal stroke, the signal from the pressure transducer 30and the signal from the pedal transducer 19 can be given equal weight indetermining the brake demand signal.

The pressure signal from the pressure transducer 30 is proportional tothe force exerted by the driver on the pedal 14. Instead of using apressure transducer to measure pressure resulting from the force exertedby the driver on the brake pedal 14, it is contemplated that a directmeasurement of the force upon the brake pedal may be obtained by use ofa strain gauge suitably positioned in the linkage extending from thebrake pedal 14 to the pistons of the master cylinder 12. This measuremay be used in developing a brake demand signal instead of the signalfrom the pressure transducer 30.

One preferred embodiment of an algorithm for a brake demand signaldevelops a signal P_(BBW), which represents the pressure at which thenormal source 4 is being commanded by the driver to deliver hydraulicfluid pressure to the brakes 11a, b, c, and d. This signal may beoverridden by such automatic controls as collision avoidance signals orantilock braking control signals. P_(BBW) is developed from a travelcommand component, P_(CMD) _(—) _(TRAVEL), and a force commandcomponent, P_(CMD) _(—) _(FORCE). The force command component P_(CMD)_(—) _(FORCE) is developed from the pressure signal from the pressuretransducer 30 (or a force sensor, as discussed above). P_(CMD) _(—)_(TRAVEL) and P_(CMD) _(—) _(FORCE) are conditioned for backlash(hysterisis) and subjected to limits prior to being input to developP_(BBW).

P_(CMD) _(—) _(Travel) has both proportional and squared functions, asindicated by the following equation:P_(CMD) _(—) _(TRAVEL)=P_(T)×k₁+P² _(T)×k₂  (1)

where P_(T) is the conditioned signal from the displacement transducer19, and k₁ and k₂ are gain factors constants which may be suitablyadjusted to further condition P_(CMD) _(—) _(TRAVEL). P_(CMD) _(—)_(FORCE) also has both proportional and squared functions, as indicatedby the following equation:P_(CMD) _(—) _(FORCE)=P_(F)×k₃+P² _(F)×k₄  (2)

where P_(F) is the conditioned signal from the pressure transducer 30,and k₃ and k₄ are gain factors constants which may be suitably adjustedto further condition P_(CMD) _(—) _(FORCE).

P_(CMD) _(—) _(FORCE) and P_(CMD) _(—) _(TRAVEL) as developed inequations 1 and 2 above are blended to develop P_(BBW) according to thefollowing two equations (3 and 4):W_(BLEND)=P_(F)×k_(BLEND)−P_(BLEND) _(—) _(OFFSET)|^(high)_(low)limit  (3)P_(BBW)=P_(CMD) _(—) _(TRAVEL)×(1−W_(BLEND))+P_(CMD) _(—)_(FORCE)×W_(BLEND)  (4)

With this system, one measures the drivers intent through pedal traveland force “electrically”. These signals are electrically blended toprovide a desired command to the normal source 4. The output is appliedthrough a resolution circuit (not shown) which sets a limitation on thesignal to control the minimum step of change to limit hunting and noise.The signal is further conditioned in a slew circuit to limit the rate ofcommanded pressure apply. The signal is further subjected to limits interms of the maximum pressure which can be commanded. If pedal traveland force are both at minimum, a default negative pressure commandsignal is preferably switched in to force P_(BBW) to a negative valve.This insures that the pressure control valve of the normal source 4(discussed in detail below) smoothly transitions to a zero pressure outcondition during a pressure reduction cycle before the spool of thepressure control valve is “parked”, and avoiding “hunting” or“simmering” of the control valve due to noise in the circuitry whenthere is no actual demand signal.

As the operator of the vehicle depresses the brake pedal 14, the mastercylinder 12 is actuated, thereby causing an increase in pressure withinthe conduits 16 and 17. The increase pressure within the conduit 16compresses the spring of the pedal simulator 26, and the pressure in theconduit 16 is sensed by the pressure transducer 30. The pedal simulator26 is provided so that the operator of the vehicle experiences aconsistent pedal feel, whether or not the isolation valves 22a and 22bare closed. It is also contemplated that the simulator valve 28 may beomitted. If the simulator valve 28 is omitted, the master cylinder 12should pressurize a sufficient volume of brake fluid to supply both thepedal simulator 26 and actuate the vehicle brakes 11a and 11b withadequate pressure in the event of a failure of the normal source 4.

The pressure in the conduits 16 and 17 between each isolation valve 22aand 22b, and the respective vehicle brake 11a and 11b, is sensed byrespective pressure transducers 36a and 36b, which supply signalsrepresentative of the respective sensed pressures to the control module10. The control module 10 utilizes the pressure signals produced by thepressure transducers 36a and 36b for purposes which will be describedbelow. As also will be further described below, the control module 10controls the operation of the simulator valve 28 and the isolationvalves 22a and 22b.

As indicated above, the isolation valves 22a and 22b are energized andshut during normal operation of the brake system 2. Only in an abnormalsituation, such as a loss of electrical power, will the isolation valves22a and 22b remain open after the driver initiates a brake demand signalby depressing the brake pedal 14. In such a situation, the mastercylinder 12 acts to supply pressurized hydraulic brake fluid to thevehicle brakes 11a and 11b through the open isolation valves 22a and22b. However, absent some type of failure, it is intended that thenormal source 4 should supply pressurized hydraulic brake fluid foractuating the vehicle brakes 11a, b, c, and d.

The normal source 4 includes a pump 42 which is capable of pumpinghydraulic brake fluid from the reservoir 20 to actuate the vehiclebrakes 11a, b, c, and d. The pump 42 is preferably electrically drivenby a motor 43 under the control of the control module 10. However, thepump 42 may be driven by any suitable means, with the output of the pump42 being controlled by the control module 10. The normal source 4 isprovided with over-pressure protection by a relief valve 44 which openswhen a preset pressure is exceeded to direct pressurized brake fluidfrom the discharge of the pump 42 back to the reservoir 20.

Pressurized hydraulic brake fluid from the pump 42 is supplied to a highpressure accumulator 46 through a check valve 47. The check valve 47allows brake fluid to flow from the discharge of the pump 42 andrestricts brake fluid from flowing into the pump 42 through thedischarge port. The accumulator 46 is conventional, including a pistonmovable with a sliding seal within the cylinder of the accumulator 46,and a pre-charge of nitrogen acting as a spring element. Other suitablespring elements which are contemplated include a compressible volume ofany other suitable gas, a metallic or elastomeric spring, or otherspring arrangement. The pre-charge of nitrogen contained in theaccumulator 46 biases the piston toward the fluid connection of theaccumulator 46. Of course, any suitable accumulator design may be used,and the accumulator 46 need not be of the piston design depicted. Forexample, the accumulator 46 may be of the diaphragm type, with adiaphragm or bellows made of metal, rubber, or plastic or otherelastomer.

As pressurized hydraulic brake fluid flows into the accumulator 46through the fluid connection, the piston of the accumulator 46 is movedto further compress the nitrogen gas precharge. In this condition, theaccumulator 46 contains a reservoir of hydraulic brake fluid which ispressurized by the piston under the influence of the compressed nitrogengas, which may be used to actuate the vehicle brakes 11a, b, c, and dwhether or not the pump 42 is running. The pressure of the hydraulicbrake fluid in the accumulator 46 is sensed by a pressure transducer 49,which supplies a corresponding signal to the control module 10.

The normal source 4 also includes a pressure isolation valve 48. Thepressure isolation valve 48 is controlled by the control module 10 tomove between a de-energized position, shown in FIG. 1, in whichpressurized brake fluid in the accumulator 46 is prevented fromdischarging from the accumulator 46, and an energized position in whichpressurized brake fluid can flow out of the accumulator 46. The pressureisolation valve 48 will normally be deenergized closed to preventdischarge of the accumulator 46 due to system leakage past various othersystem valves. Note that the high pressure relief valve 44 and the checkvalve 47 cooperate with the pressure isolation valve 48 to prevent thefluid within the accumulator 46 from discharging when the pressureisolation valve 48 is shut. When braking is required, the pressureisolation valve 48 is energized open to allow the pressurized hydraulicbrake fluid in the accumulator 46 to be used to apply the vehicle brakes11a, b, c, and d. The location of the pressure isolation valve 48 in thebrake system 2 provides for over-pressure protection for the accumulator46 by the relief valve 44.

Through the pressure isolation valve 48, the outlet of the pump 42 andthe accumulator 46 are in fluid communication with a fluid conduit 50.The fluid conduit 50 is in fluid communication with proportional controlvalves 51a, b, c, and d. A filter 52 is preferably provided in the fluidconduit 50 between the sources of pressurized hydraulic brake fluid (thepump 42 and the accumulator 46) and the proportional control valves 51a,b, c, and d to remove contaminating particles from the hydraulic brakefluid supplied to the proportional control valves 51a, b, c, and d.

The illustrated proportional control valve 51 a has a port which is influid communication with a fluid separator unit 54a. The proportionalcontrol valve 51b has a port which is in fluid communication with afluid separator unit 54b. The fluid separator unit 54b is similar instructure and function to the fluid separator unit 54a. As shown in moredetail in FIG. 7, the fluid separator unit 54a includes a housing 55with a cylindrical bore 55a therethrough. A first end 55b of the bore55a is in fluid communication with the proportional control valve 51a. Asecond end 55c of the bore 55a is in fluid communication with thevehicle brake 11a.

A fluid separator piston 56 is slideably disposed within the cylindricalbore 55a between the first end 55b and the second end 55c of the bore55a. The piston 56 is generally cylindrical, having a first piston face56a in fluid communication with the normal source 4 via the first end55b of the bore 55a and a second piston face 56b in fluid communicationwith the backup source 6 via the second end 55c of the bore 55a. Thepiston 56 is preferably formed with a pair of axially spaced apart,circumferentially extending grooves 56c and 56d. The groove 56c isformed near the first piston face 56a, while the groove 56d is formednear the second piston face 56b. The piston 56 is further formed with areduced diameter projection 56e extending axially from the second pistonface 56b. Preferably the piston 56 is also formed with a raised boss 56fon the first piston face 56a. The boss 56f assists in preventing ahydraulic lock between the piston 56 and the adjacent end wall of thebore 55a when the piston 56 is in the unactuated position shown in FIG.7

A first seal 57a, which is preferably a lip seal, is disposed in thefirst groove 56c formed in the piston 56 and oriented to slidingly sealbetween the piston 56 and the wall of the bore 55a against pressurizedhydraulic brake fluid from the normal source 4 supplied to the first end55b of the bore 55. The first seal 57a and the piston face 56a cooperateto define a first working face of the piston 56.

Similarly, a second seal 57b, which is also preferably a lip seal, isdisposed in the second groove 56d formed in the piston 56. The secondseal 57b is oriented to slidingly seal between the piston 56 and thewall of the bore 55a against pressurized hydraulic brake fluid from thebackup source 6 at the second end 55c of the bore 55. The second seal57b and the piston face 56b, including the extension 56e, cooperate todefine a second working face of the piston 56.

It will be appreciated from FIG. 7 that the diameter of the piston 56 isthe same in the region of the seal 57a as it is in the region of theseal 57b. Thus, the cross-sectional area of the first working face ofthe piston 56 (the area acted upon by the adjacent volume of hydraulicbrake fluid) is the same as the cross-sectional area of the secondworking face of the piston 56. Furthermore, the bore 55a is of constantdiameter. These features of the invention are believed to simplifyconstruction of the fluid separator unit 54a and reduce costs comparedto a possible alternate construction having a stepped bore and steppedpiston sliding therein. In the fluid separator unit 54a, pressurizedfluid from the backup source 6 normal source 4 actuates the piston 56 ofthe fluid separator unit 54a to pressurize the trapped hydraulic brakefluid between the isolation valve 22a and the wheel brake 11a tosubstantially the same pressure as the pressure at which the hydraulicbrake fluid is supplied to the fluid separator unit 54a from the backupsource 6 normal source 4. Any differences due to the compression of thespring 58 of the fluid separator unit 54a and friction are generallynegligible fractions of the pressures of the hydraulic brake fluidacting in the fluid separator unit 54a during braking.

The fluid separator unit 54a permits pressure in the hydraulic brakefluid on one side of the piston 56 (acting on one of the first andsecond working faces of the piston 56) to be transferred to thehydraulic brake fluid on the other side of the fluid separator piston 56(acting on the other of the first and second working faces of the piston56) through movement of the fluid separator piston 56 within the bore55a. The fluid separator unit 54a is sealed to the wall of the bore 55aby the seals 57a and 57b to prevent intermixing of the hydraulic brakefluids on either side of the piston 56. As will become apparent, aprimary purpose of the fluid separator unit 54a (and of the fluidseparator unit 54b) is to maintain the integrity and operability of thebackup source 6 of hydraulic brake fluid even in the event of amalfunction or rupture of the normal source 4.

A spring 58 is provided which biases the fluid separator piston 56toward the unactuated position of the piston 56, at the first end 55b ofthe bore 55a of the fluid separator unit 54a. The fluid separator piston56 is constrained to remain in the bore 55a, and thus a complete loss ofhydraulic brake fluid and pressure on one side of the fluid separatorpiston 56 of the fluid separator unit 54a will not result in loss offluid or complete loss of pressure on the other side of the fluidseparator piston 56. As pressurized hydraulic brake fluid flows into thefluid separator unit 54a from the proportional control valve 51a, thefluid separator piston 56 is moved to an actuated position, compressingthe spring 58. The piston 56 acts on the hydraulic brake fluid in thesecond end 55c of the bore 55, thereby pressurizing the hydraulic brakefluid trapped between the energized isolation valve 22a and the vehiclebrake 11a and causing the vehicle brake 11a to be applied. The normalsource 4 also includes a fluid separator unit 54b connected (in anarrangement similar to that of the fluid separator unit 54a, the controlvalve 51 a and the brake 11a) between the control valves valve 51b andthe vehicle brake 11b. The fluid separator unit 54b is similar inconstruction and operation to the fluid separator unit 54a.

FIGS. 8 and 9 illustrate a piston 59 which is an alternate embodiment ofa piston which can be used in the fluid separator units 54a and 54b inlieu of the piston 56. As shown therein, the piston 59 is a generallycup-shaped cylindrical piston, having a first piston face 59a in fluidcommunication with the normal source 4 via the first end 55b of the bore55a and a second piston face 59b in fluid communication with the backupsource 6 via the second end 55c of the bore 55a. The piston 59 ispreferably formed with a pair of axially spaced apart, circumferentiallyextending grooves 59c and 59d. The groove 59c is formed near the firstpiston face 59a, while the groove 59d is formed near the second pistonface 59b. The piston 59 is further formed with a recess 59e extendingaxially into the piston 59 from the second piston face 59b.

If desire desired, a groove 59f may be defined in the first piston face59a of the piston 59. The groove 59f, like the boss 56f, assists inpreventing hydraulic locking of the piston 59 at the unactuated positionthereof. The groove 59f may be formed to extend only partially acrossthe first face 59a of the piston 59, and still be effective inpreventing hydraulic locking of the piston 59.

A first seal (not shown), which is preferably an o-ring, is disposed inthe first groove 59c formed in the piston 59. The first seal slidinglyseals between the piston 59 and the wall of the bore 55a, scalingagainst pressurized hydraulic brake fluid from the normal source 4supplied to the first end 55b of the bore 55. The first seal and thepiston face 59a cooperate to define a first working face of the piston59.

Similarly, a second seal (not shown), which is also preferably ano-ring, is disposed in the second groove 56d 59d formed in the piston 5659. The second seal slidingly seals between the piston 56 and the wallof the bore 55a, sealing against pressurized hydraulic brake fluid fromthe backup source 6 at the second end 55c of the bore 55. The secondseal and the piston face 56b 59b, including the recess 56e 59e,cooperate to define a second working face of the piston 56 59.

A spring 60 is disposed partially in the recess 59e and acts between thepiston 59 and the end wall at the second end of the bore 55a to urge thepiston 59 to a retracted position thereof at the first end 55b of thebore 55a. In operation, the piston 59 acts similarly to the piston 56.

Each of the proportional control valves 51a, b, c, and d areelectrically positioned by the control module 10. In a first energizedposition, the apply position, the proportional control valve 51a or bdirects the pressurized hydraulic brake fluid supplied to theproportional control valve 51 a or 51b from the fluid conduit 50 to theassociated fluid separator unit 54a or 54b. In a second energizedposition, the maintain position, the proportional control valve 51a or51b closes off the port thereof which is in communication with theassociated fluid separator unit 54a or 54b, thereby hydraulicallylocking the associated fluid separator piston of the fluid separatorunit 54a or 54b in a selected position. In a de-energized position, therelease position, the spool of the proportional control valve 51a or 51bis moved by a spring to the position illustrated in FIG. 1, where theproportional control valve 51a or 51b provides fluid communicationbetween the associated fluid separator unit 54a or 54b and the reservoir20. This vents pressure from the associated fluid separator unit 54a or54b, allowing the piston 56 thereof to move back to the unactuatedposition thereof under the urging of the associated spring 58, therebyreducing pressure at the associated vehicle brake 11a or 11b. Theproportional control valves 51c and 51d generally operate in the samemanner as the proportional control valves 51a and 51b, except that thereis not a fluid separator unit positioned between the proportionalcontrol valves 51c and 51d and the respective vehicle brakes 11c and 11dsince the backup source 6 does not supply the vehicle brakes 11c and11d. The pressures in the conduits between each proportional controlvalve 51c and 51d, and the respective vehicle brake 11c and 11d, issensed by respective pressure transducers 36c and 36d, which supplysignals representative of the respective sensed pressures to the controlmodule 10.

Preferably, the positions of the proportional control valves 51a, b, c,and d are controlled so that the controlled pressures are proportionalto the current of the energizing electrical signal. The controlledpressure for the proportional control valves 51a or 51b is the fluidpressure in the fluid conduit between the respective proportionalcontrol valve 51a or 51b and the associated fluid separator unit 54a or54b. The controlled pressure for the proportional control valves 51c or51d is the fluid pressure in the fluid conduit between the respectiveproportional control valve 51c or 51d and the associated vehicle brake11c or 11d. A respective pressure feedback conduit 61a, b, c, or d isprovided to the associated proportional control valve 51a, b, c, or d,so that controlled pressure opposes the movement caused in theproportional control valve 51a, b, c, or d caused by increasingenergization of the solenoid thereof.

It may be desirable, however, to control the position of theproportional control valves 51a, b, c, and d, such that the exactposition of a proportional control valve 51a, b, c, or d is proportionalto the energizing electrical signal from the control module 10. Thus,the proportional control valves 51a, b, c, or d may be positioned at aninfinite number of positions rather than just the three positionsdescribed above. In other words, the proportional valves 51a, b, c, or dmay be positioned in the apply position, the maintain position, or therelease position; the proportional valves 51a, b, c, or d may also bepositioned to any position between the apply and maintain position toprovide a throttled path for directing the pressurized hydraulic brakefluid to the associated fluid separator unit 54a, b, c, or d, and theproportional valves 51a, b, c, or d may be positioned to any positionbetween the release position and the maintain position to provide athrottled path for venting the pressurized hydraulic brake fluid fromthe associated fluid separator unit 54a, b, c, or d to the reservoir 20.If it is desired to rapidly apply pressurized hydraulic brake fluid tothe associated vehicle brake 11a, b, c, or d, the proportional controlvalve 51a, b, c, or d is moved fully to the first energized (apply)position. However, if it is desired to more slowly apply hydraulic brakefluid to the associated vehicle brake 11a, b, c, or d, the proportionalcontrol valve 51a, b, c, or d is moved to a position between the first(apply) and second (maintain) energized positions described above, sothat pressurized hydraulic brake fluid can be applied to the associatedvehicle brake 11a, b, c, or d at less than the maximum rate possiblebecause the proportional control valve 51a, b, c, or d is throttled.Similarly, the proportional control valve 51a, b, c, or d may be movedto a position between the second (maintain) energized position and thede-energized position to vent pressurized hydraulic brake fluid from theassociated vehicle brakes 11a, b, c, or d at less than the rate possiblewhen the proportional control valve 51a, b, c, or d is in thede-energized (release) position.

The brake system 2 further includes a pair of normally open balancevalves 62 and 64 which are electrically controlled by the control module10. The balance valve 62 selectively isolates the fluid communicationbetween the outlet ports of the proportional control valves 51a and 51b.The balance valve 64 selectively isolates the fluid communicationbetween the vehicle brakes 11c and 11d. As will be discussed in detailbelow, another function of the balance valves 62 and 64 is to providefor calibration between the vehicle brakes 11a and 11b, and between thevehicle brakes 11c and 11d, respectively.

During normal braking, the control module 10 maintains the isolationvalves 22a and 22b energized shut and the simulator valve 28 energizedopen, thereby isolating the master cylinder 12 from the vehicle brakes11a and 11b, and hydraulically connecting the pedal simulator 26 to themaster cylinder 12. Fixed volumes of hydraulic brake fluid are trappedbetween the isolation valve 22a and the vehicle brake 11a, and betweenthe isolation valve 22b and the vehicle brake 11b. The pump 42 issuitably run to cooperate with the accumulator 46 to supply sufficientquantities of pressurized hydraulic brake fluid to meet the brakedemand. Generally, the pump 42 is shut off by the control module 10 whena sufficient quantity of suitably pressurized hydraulic brake fluid hasbeen generated to meet brake demand. In this manner, the fluid conduit50 is pressurized up to the proportional control valves 5a, b, c, and d.

The pressure transducer 49 monitors the pressure in the accumulator 46and the fluid conduit 50 (when the pressure isolation valve 48 isenergized open), providing input to the control module 10. The controlmodule 10 controls the operation of the pump 42 as needed to maintainpressure of the hydraulic brake fluid of the normal source 4. Suitablythe control module 10 may be designed to alert the vehicle operator ifthe pressure response is not as expected.

In the event that an abnormal loss of pressure in the normal source 4,or other failure of the normal source 4, the control module 10 monitorsthe pressure transducer 49, 36a, 36b, 36c, 36d and 30 to attempt todetermine the extent of the abnormality. Pre-programmed degraded controlschemes are preferably programmed into the control module 10. As will bediscussed below, the control module 10 may maintain braking control fromthe normal source 4 in certain degraded conditions. In certain otherconditions, the control module 10 may cause pressurized hydraulic brakefluid for operation of the vehicle brakes 11a and 11b to be suppliedfrom the manual backup source 6, from the master cylinder 12. In thiscase, the isolation valves 22a and 22b, the simulator valve 28, and theproportional control valves 5a, b, c, and d are deenergized, therebyconnecting the vehicle brakes 11a and 11b to the master cylinder 12 formanual control. Note that even a rupture of the fluid conduit 50 of thenormal source 4, and a complete draining of hydraulic brake fluid fromthe normal source 4, will not prevent the operation of the vehiclebrakes 11a and 11b by the master cylinder 12, since the fluid separatorunits 54a and 54b will prevent any loss of hydraulic brake fluid fromthe conduit 16 or the conduit 17 of the backup source 6 to the piping ofthe normal source 4.

During normal braking, however, with the normal source 4 available, theoperator of the vehicle generates a manual brake demand signal bydepressing the brake pedal 14. Depressing the brake pedal 14 sendspressurized hydraulic brake fluid to the pedal simulator 26. Thepressure of the hydraulic brake fluid in the pedal simulator 26increases as the brake pedal 14 is further depressed, owing to furthercompression of the spring 26e of the pedal simulator 26. The resultantrise in pressure in the conduit 16 is monitored by the pressuretransducer 30. As indicated above, the output signal of the pressuretransducer 30 is a brake demand signal sent to the control module 10.The more the brake pedal 14 is depressed, the greater the brake demandsignal developed by the pressure transducer 30. Similarly, the more thebrake pedal 14 is depressed, the greater the brake demand signalgenerated by the brake pedal displacement transducer 19 which is sent tothe control module 10. As described above, the brake demand signalsgenerated by the displacement transducer 19 and the pressure transducer30 are combined to generate a system brake demand signal.

Various automated brake demand signals and brake modulation signals maybe supplied to the control module 10. For example, it may be desired toactuate one or more of the vehicle brakes 11a, b, c, and d for purposesof traction control, coordinated vehicle stability control, hill hold,or automated collision avoidance control schemes, even when the vehicleoperator is not depressing the brake pedal 14. Similarly, it may bedesired to temporarily decrease the braking force of one or more of thevehicle brakes 11a, b, c, and d for the purposes of antilock brakingeven if the operator is depressing the brake pedal 14. Signals which maybe supplied to the control module 10 for the purposes of such automatedcontrol schemes may include wheel speed of each of the vehicle's wheels,vehicle deceleration, steering angle, vehicle yaw rate, vehicle speed,vehicle roll rate, and signals from radar, infrared, ultrasonic, orsimilar collision avoidance systems, cruise control systems (includingAICC—Autonomous Intelligent Cruise Control Systems), and the like. Itmay also be desirable to actuate one or more of the vehicle brakes 11a,b, c, and d for purposes of panic brake assist when the vehicle operatoris depressing the brake pedal 14.

When braking is demanded at one or more of the vehicle brakes 11a, b, c,and d, the pressure isolation valve 48 is opened, and the appropriateproportional control valve(s) 51a, b, c, and d are energized to an applyposition. The balance valves 62 and 64 are normally actuated to a closedposition during braking, thereby isolating the vehicle brakes 11a, b, c,and d from each other. For the vehicle brakes 11a and 11b, pressurizedhydraulic brake fluid from the normal source 4 is applied to the fluidseparator piston(s) 56 of the respective fluid separator unit(s) 54a and54b, causing the fluid separator piston(s) 56 to move toward the secondend 55c of the bore 55a, compressing the spring 58, and forcingpressurized hydraulic brake fluid out of the second end 55c of the fluidseparator unit(s) 54a and 54b. Since there is already a trapped volumeof hydraulic brake fluid between the vehicle brakes 11a and 11b and theassociated isolation valve 22a and 22b, the pressurized hydraulic brakefluid from the fluid separator unit(s) 54a and 54b causes the associatedvehicle brake(s) 11a and 11b to be applied. Since there are no fluidseparator units associated with the vehicle brakes 11c and 11d,pressurized hydraulic brake fluid from the proportional control valves51c and 51d, respectively, is applied to the associated vehicle brakes11c and 11d. Of course, fluid separator units could suitably be addedbetween the proportional control valves 51c and 51d and the associatedvehicle brakes 11c and 11d together with selective fluid communicationwith the master cylinder 12 if it is desired to provide manual brakingto the rear vehicle brakes 11c and 11d.

The pressure of the hydraulic brake fluid applied to the vehicle brakes11a, b, c, and d is monitored by the associated pressure transducers36a, b, c, and d. When a desired brake pressure is reached in a vehiclebrake 11a, b, c, or d, the control module 10 will cause the associatedproportional control valve 51a, b, c, or d to move to the maintainposition, to hold the desired pressure. If the accumulator 46 is unableto supply sufficient pressure and volume of pressurized hydraulic brakefluid to the proportional control valves 5a, b, c, and d, the pump 42 isstarted to supply the needed pressurized hydraulic brake fluid.

When the pressure at the vehicle brake 11a, b, c, or d is no longer thedesired pressure, the control module 10 will position the associatedproportional control valve 51a, b, c, or d to apply more pressurizedfluid to increase the pressure applied, or to vent pressurized brakefluid to the reservoir 20 to decrease or release the pressure applied,as appropriate, in response to the varying brake and modulation demandsignals and the control scheme programmed into the control module 10.

After installation of the brake system 2 or during periodic intervals,the brake system 2 can be calibrated to determine the zero point readingfor each of the pressure transducers 36a, b, c, and d. During anon-braking situation, in which the brake pedal 14 is not depressed andthe master cylinder 12 and the normal source 4 are not actuated, thebalance valves 62 and 64 are left in their unactauted open position. Afirst reading is taken of each pressure transducer 36a, b, c, and d todetermine a zero reference value. After the first reading has beenrecorded, the balance valves 62 and 64 are actuated to a closedposition. The normal source 4 is then actuated and the proportionalvalves 51a, b, c, and d are energized to an apply position to increasethe pressure within the fluid conduits from the proportional controlvalves 51a, b, c, and d to the associated vehicle brakes 11a, b, c, andd. The normal source 4 and the proportional control valves 51a, b, c,and d can be actuated by a calibrating command signal from the controlmodule 10 or by depressing the brake pedal 14 to actuate the brakesystem 2 as described above. A second reading is taken of each pressuretransducer 36a, b, c, and d to determine a signal gain. Preferably, thesecond reading is taken after the pressure increase generally levels outso that the fluid flow effects of the brake fluid through the brakesystem 2 do not adversely affect the reading. If desired, severalpressure readings can be taken at different pressure levels to check thelinearity of the response for each of the pressure transducers 36a, b,c, and d.

The calibration method described above operates under the assumptionthat the proportional control valves 51a, b, c, and d are functioningproperly. This assumption can be generally verified by checking that thepressure readings of the pressure transducers 36a, b, c, and d arewithin an expected value range. If the readings of the pressuretransducers 36a, b, c, and d are not within an expected value range, thebrake system 2 can be further analyzed by first deenergizing the balancevalves 62 and 64 to an open position. The proportional control valves51a, b, c, and d are then operated individually and the readings of thepressure transducers 36a, b, c, and d are monitored to determine whichwheel fluid circuit may be faulty.

The balance valves 62 and 64 are normally in a non-actuated openposition during a non-braking condition, thereby conserving or reducingcurrent consumption of the brake system 2.

The balance valves 62 and 64 also provide fail-safe backup in certainconditions of system failure. If a proportional control valve 51a, b, c,or d is not properly supplying the required pressure to its associatedvehicle brake 11a, b, c, or d, the associated balance valve 62 or 64 canbe opened so that the other proportional control valve 51a, b, c, or dof an associated pair can actuate the vehicle brake 11a, b, c, or d. Forexample, if the vehicle brake 11a was not receiving pressure from thenormal source 4, such as by failure of the proportional control valve51a or a rupture within the piping from the normal source 4 to thevehicle proportional control valve 51a, the balance valve 62 can bedeenergized to an open position. The proportional control valve 51b canthen be used to supply pressurized fluid to both of the vehicle brakes11a and 11b. Similarly, if the vehicle brake 11b failed, theproportional control valve 51a can be used to supply pressure to thevehicle brake 11b. Likewise, for the pair of vehicle brakes 11c and 11d,the balance valve 64 can be deenergized to an open position so that theunfailed proportional control valve 51c or 51d can supply pressure toboth vehicle brakes 11c and 11d.

In another failure scenario, if a pressure transducer 36c or 36d sensesa pressure drop (such as caused by a rupture in one of the fluid linesconnected to the vehicle brake 11c or 11d), the balance valve 64 can beactuated to remain in a closed position even when no braking is inprogress so that air cannot enter both of the fluid lines supplyingpressurized fluid to the remaining operable vehicle brake 11c or 11d. Inthe event that an abnormal pressure drop is detected between one of thefluid separator units 54a or 54b and the respective pressure controlvalve 51a or 51b, the balance valve 62 can be actuated to a closedposition during periods of braking and non-braking so that air cannotenter both of the fluid lines supplying pressurized fluid to theremaining operable vehicle brake 11a or 11b. Note, if an abnormalpressure drop is detected on the other side of a fluid separator unit54a or 54b (in the fluid lines between a fluid separator unit 54a or 54band the respective vehicle brake 11a or 11b), the appropriate isolationvalve 22a or 22b is actuated to a closed position.

Note that even a rupture of the fluid conduit 50 of the normal source 4,and a complete draining of hydraulic brake fluid from the normal source4 will not prevent the operation of the vehicle brakes 11a and 11b bythe master cylinder 12, since the fluid separator units 54a and 54b willprevent any loss of hydraulic brake fluid from the conduit 16 or theconduit 17 to the normal source 4.

It should be noted that many of the components described and illustratedas discrete components may be easily combined in a single compacthousing. For example, the master cylinder 12, the isolation valves 22aand 22b, the simulator valve 28, the pedal simulator 26 and one or moretravel transducers and one or more pressure transducers 30, could beintegrated into one unit with or without the reservoir 20 includedtherein. Similarly, the fluid separator units 54a and 54b, theproportional control valves 51a, b, c, and d, the balance valves 62 and64, the pressure transducers 36a, b, c, and d, the filter 52, and therelief valve 44 could be integrated into a single unit. The accumulator46, the pressure isolation valve 48, the relief valve 44, the pump 42with motor, and the pressure transducer 49 could be incorporated intoone unit. The control module 10 (also known as an ECU—Electronic ControlUnit) could be integrated into the unit containing the pump 42. Indeed,it is contemplated that any or all of the components discussed in thisparagraph could be highly integrated into one unit.

It is also contemplated that the fluid separator units 54a and 54b canbe integrated into their respective vehicle brake 11a and 11b (forexample, in the caliper of a disc brake or the actuator of a drumbrake).

The brake pedal detector 18 and pedal displacement transducer 19 may beintegrated into a package with the brake pedal 14 or the master cylinder12. It may also be desired to provide a pedal simulator 26 for each ofthe conduits 16 and 17.

It should also be noted that it is generally desirable to use compactcomponents to allow the brake system 2 to fit within the spaceconstraints of modem vehicle designs. Therefore, it would be desirableto use a relatively compact master cylinder 12, with the brake system 2.It is expected that power assist (e.g., vacuum or hydraulic boost) offluid pressure in the master cylinder is 12 will not be required, sincethe master cylinder 12 is not the normal source of pressurized brakefluid for actuating the vehicle brakes 11a, b, c, and d. However, ifdesired, vacuum or hydraulic boosters, or other suitable arrangementsfor increasing the force applied to operate the master cylinder 12, maybe used.

FIG. 2 illustrates a preferred embodiment of the damping circuit 29. Thedamping circuit 29 is hydraulically positioned between the pedalsimulator 26 and the simulator valve 28. The dampening circuit 29 hasthree parallel fluid branches 80, 82, and 84. The fluid branch 80includes an orifice 86 and a check valve 88. The check valve 88restricts the flow of fluid through the fluid branch 80 in a directionfrom the simulator valve 28 to the pedal simulator 26 (referred to asthe “apply direction”). The fluid branch 82 includes an orifice 90.Fluid is free to flow through the branch 82 in either the applydirection or in a direction from the pedal simulator 26 to the simulatorvalve 28 (referred to as the “release direction”). The fluid branch 84includes a relief valve 94 which prevents the flow of fluid through thefluid branch 84 in the apply direction until a predetermined pressure isreached. When the predetermined pressure is reached, the relief valve 94opens, thereby allowing fluid to travel through the fluid branch 84 inthe apply direction. It will thus be apparent that when fluid is flowingin the release direction (“release direction flow”), the fluid can flowthrough both the fluid branches 80 and 82. In contrast, of the fluidbranches 80 and 82, fluid can only s flow into the pedal simulator 26(“apply direction flow”) through the fluid branch 82.

Referring to FIGS. 1 and 2, as the operator of the vehicle depresses thebrake pedal 14 and actuates the master cylinder 12, the pressure withinthe conduit 16 is increased. During normal braking, in which brakefailure has not occurred, the simulator valve 28 is open and theisolation valves 22a and 22b are closed. Fluid flows through thedampening circuit 29 via the fluid branch 82 into the pedal simulator26. As long as the pressure within the conduit 16 is less than thepredetermined pressure which opens the relief valve 94, all of the fluidtraveling through the dampening circuit 29 will be directed through thefluid branch 82. As the fluid travels through the restrictedcross-sectional area of the orifice 90, the operator of the brake pedalwill feel a resistance to this fluid movement. Of course, the operatorof the vehicle will also feel a resistance force acting on the brakepedal 14 due to such factors as the compression of the spring 26e withinthe pedal simulator 26, and friction of the moving components of thebrake system 2. The combination of the dampening circuit 29 and thepedal simulator 26, and the other factors mentioned above, create apedal feel characteristic which can closely mimic that of a conventionalbrake system or other desired pedal feel.

If during brake apply the pressure within the conduit 16 is greater thanthe predetermined pressure which opens the relief valve 94, such asduring a panic brake situation, the relief valve 94 will open, therebyallowing a greater amount of fluid to travel through the dampeningcircuit 29 in the apply direction.

Upon release of the brake pedal 14, the fluid will flow out of the pedalsimulator 26 and through the dampening circuit 29 via the fluid branches80 and 82. Unlike the case during brake apply, fluid flows through boththe orifice 90 and the orifice 86. Thus, the pedal response felt by theoperator is different in the brake release direction than in the brakeapply direction.

For the brake apply direction, the pressure difference between themaster cylinder 12 and the pedal simulator 26 depends upon the flow offluid through the orifice 90 and possibly through the relief valve 94.For the brake release direction, the pressure difference between themaster cylinder 12 and the pedal simulator 26 depends upon the flowthrough the orifices 86 and 90. The flow volume is related to theactuation speed of the brake pedal 14. Thus, the resistancecharacteristic is dependent on the actuation speed of the brake pedal aswell as the pressure difference between the master cylinder 12 and thepedal simulator 26. The resistance characteristic can be altered byadjusting the predetermined pressure which opens the relief valve 94 andby adjusting the cross-sectional areas of the orifices 86 and 90.Preferably, the orifices 86 and 90 are sized or adjustable so thatactuation of the brake pedal 14 (apply direction flow) creates more of aresistance than when the brake pedal 14 is released (release directionflow).

The total cross sectional area for apply direction flow through thebranches 80 and 82 during normal brake apply situations (in which therelief valve 94 remains shut) is less than the cross sectional area forrelease direction flow. This has been found to be a significant factorto achieving a good pedal feel. Generally, a preferred ratio of crosssection area for release flow to the cross sectional area for applydirection flow through the branches 80 and 82 has been found to begreater than unity (1:1) and less than about 10:1, and most preferablyin the range of about 2:1 to 4:1.

As previously stated above, the fluid branch 84 includes the reliefvalve 94 which prevents the flow of fluid through the fluid branch 84 inthe apply direction until a predetermined pressure is reached. When thepredetermined pressure is reached, the relief valve 94 opens, therebyallowing fluid to travel through the fluid branch 84 in the applydirection. This serves to limit the pedal reaction force a driver of thevehicle experiences during a panic stop situation. The value of thesuitable predetermined pressure is dependent upon the area of theworking face of the piston in the master cylinder 12. If the workingface area of the piston in the master cylinder is larger, for a givenpressure seen at the master cylinder 12 as a result of pressure dropacross the damping circuit 29, the reaction force on the brake pedal 14is correspondingly larger. However, for vehicles such as light trucksand passenger cars, with the master cylinder 12 being of a sizetypically supplied on such vehicles, a preferred range (for good pedalfeel) for the predetermined pressure setpoint for operating the reliefvalve 94 has been found to be from about 5 bar to about 30 bar.

Although FIG. 2 illustrates a specific example of a dampening circuit29, it should be understood that any suitable configuration can be used.

There is also shown schematically in FIG. 2 an example of an expansionvolume unit 31 which is in fluid communication with the pedal simulator26. The expansion volume unit 31 preferably includes a flexible membranedisposed within a cylinder. The membrane is preferably made of anelastomeric material. As the brake pedal 14 is depressed, pressurizedfluid from the master cylinder 12 is directed into the expansion volumeunit 31, thereby expanding the membrane in an outwardly direction.Preferably, the membrane expands into a caged housing ultimately whichlimits the expansion of the membrane. As the membrane expands, themembrane provides increasing resistance to further expansion, resultingin a gradually increasing pressure in the conduit 16 as fluid flows fromthe master cylinder 12 into the expansion volume unit 31. Thisresistance to expansion is fed back to the brake pedal 14 through theincrease in pressure of the conduit 26 reacting in the master cylinder12, so that the operator of the brake pedal 14 feels an increasedresistance. The membrane will continue to expand outwardly until themembrane expands to the boundaries of the caged member. The resistanceforce caused by the expansion of the membrane is dependent on thevarious design criteria of the expansion volume unit 31, such as thestiffness of the membrane material and the shape of the membrane andcaged housing

Although FIG. 2 illustrates a specific example of an expansion volumeunit 31, it should be understood that any suitable configuration can beused. For example, the expansion volume unit can be designed without acaged housing, and instead include a sealed chamber. The sealed chambercan be filled with air or other suitable gases to provide for areactionary spring force acting against the membrane. The sealed chambercan then be provided with a valve arrangement to seal air within thesealed chamber. The expansion volume unit 31 can also be provided with amechanical spring element to supplement the force exerted by themembrane. In a preferred embodiment, the expansion volume unit includesa housing (not shown) defining a cylinder which is vented at first end(preferably to the reservoir 20, although the cylinder may be vented toatmosphere). The second end of the cylinder of the expansion volume unitis in fluid communication with the pedal simulator 26. A piston (notshown) is slideably disposed in the cylinder of the expansion volumeunit and seals against the wall of the cylinder. A first end of thepiston is in fluid communication with the pedal simulator 26. A spring(not shown) extends between a second end of the piston of the expansionvolume unit, and a spring seat formed at the second (vented) end of thecylinder. A membrane is fixed to the first end of the piston with an airvolume formed between the membrane and the first end of the piston. Avent hole is formed through the piston to vent the air volume to thesecond (vented) end of the cylinder. As pressure increases in the pedalsimulator, the membrane in the expansion volume unit first distortsagainst the piston to collapse the air volume As pressure continues torise, the piston in the expansion volume unit (which is relativelylightly spring-loaded compared to the piston of the pedal simulator 26)begins to move, compressing the spring of the expansion volume unit. Asthe piston in the expansion volume unit nears the end of travel, thepressure in the pedal simulator 26 rises to the pressure required forthe piston in the pedal simulator 26 to begin moving. This arrangement,in a manner similar to the expansion volume unit 31, results in improvedpedal feel, as will be explained below with reference to the illustratedexpansion volume unit 31.

The expansion volume unit 31 provides for improved pedal feel duringinitial stroke movement of the brake pedal 14. FIG. 3 is a graph whichplots the input force acting on the pedal 14 by the operator of thevehicle (Pedal Force) vs. the travel length or stroke of the brake pedal14 (Pedal Travel). The plot for a typical electro-hydraulic brake systemwithout an expansion volume unit 31 is shown in a solid line. The plotfor an electro-hydraulic brake system with an expansion volume unit 31is shown in broken line.

Referring now to the solid line plot for a typical electro-hydraulicbrake system without an expansion volume unit 31, the initial stroke ofthe brake pedal 14, labeled “A” in FIG. 3, involves taking up the slackin the mechanical pedal linkages. For a conventional master cylinderhaving compensation ports, the initial stroke “A” of the brake pedal 14also involves the movement of the pistons within the master cylinder 12prior to the closing of the compensation ports by the piston seals.During the closing of the compensation ports, indicated by the referenceletter “B”, the pedal force increases relatively rapidly over arelatively small pedal travel distance. The static friction of thepiston of the pedal simulator 26 and the preload of the spring 26e ofthe pedal simulator 26 cause a generally rapid rise in pedal force withlittle or no pedal travel, as indicated by the reference letter “C”.Once the static friction and spring preload of the pedal simulator areovercome and the piston 26c in the pedal simulator 26 begins to move(break-free pressure), continued movement of the brake pedal 14 resultsin the characteristic curve of the piston/spring arrangement of thepedal simulator 26, as indicated by the reference letter “D”.

The expansion of the membrane of the expansion volume unit 31 provides aresistance which is felt by the operator of the brake pedal 14.Preferably, the expansion volume unit 31 is designed so that theoperator of the brake pedal 14 feels this gradually increasingresistance during the beginning stages of brake pedal movement as thecompensating ports close on the master cylinder 12 and pressure rises tothe break-free pressure to get the pedal simulator piston 26c moving,thereby increasing the pedal travel required for a given pressure rise,and “smoothing” the Pedal Force vs. Pedal Travel curve, such as thatindicated by the broken line of FIG. 3.

Note that the dampening circuit 29 and the expansion volume unit 31 arepreferably both included in the brake systems of the present inventiondescribed herein. However, it is contemplated that either or both of thedampening circuit 29 and the expansion volume unit 31 can be suitablyomitted. The expansion volume unit 31 can also be designed to provide aprogressively larger resistance to movement throughout the pedal stroke,thereby acting as a pedal simulator and eliminating the need for thepedal simulator 14 from the brake system.

FIG. 4 is a schematic view illustration of another vehicle brake system,indicated generally at 200, according to the present invention. Thecomponents of the brake system 200, and their function, are in manyinstances identical in function and structure to the componentsdisclosed in the above embodiment of the brake system 2 shown in FIG. 1.Such components will be referred to using the same reference numbers asin the brake system 2. Unless otherwise indicated, where a similarlynumbered component is shown in FIG. 4 but not specifically discussed,its function and structure may be taken to be similar to that of thesimilarly numbered component of the brake system 2 of FIG. 1. Similarly,if a component is discussed or implied and not specifically shown, itsstructure and function may also be taken to be similar to the previouslydisclosed, similarly situated component of FIG. 1.

The vehicle brake system 200 may suitably be used on an automotivevehicle having four wheels and a brake for each wheel. This inventionprovides The vehicle brake system 200 is an electronically controlledbrake system for the four wheels with manual backup braking to two ofthe vehicle brakes 11a and 11b. One of the differences between the brakesystems 2 and 200, is that the balance valve 62 provides communicationbetween the vehicle brakes 11a and 11b instead of communications betweenthe outlets of the pressure control valves 51a and 51b. Thus, thebalance valve 62 of the brake system 200 is hydraulically connected onthe other side of the fluid separator units 54a and 54b from the balancevalve 62 in the vehicle brake system 2. This arrangement presentsdifferent testing capabilities for the control module 10 by permittingdirect cross connection of the wheel brake 11a and the pressuretransducer 36a with the other front wheel brake 11b and the pressuretransducer 36b, for example.

FIG. 5 is a schematic view illustration of another vehicle brake system,indicated generally at 300, according to the present invention. Thecomponents of the brake system 300, and their function, are in manyinstances identical in function and structure to the componentsdisclosed in the above embodiments of the brake systems 2 and 200 asshown in FIGS. 1 and 4, respectively. Such components will be referredto using the same reference numbers as in the brake systems 2 and 200.Unless otherwise indicated, where a similarly numbered component isshown in FIG. 5 but not specifically discussed, its function andstructure may be taken to be similar to that of the similarly numberedcomponent of the brake systems 2 or 200. Similarly, if a component isdiscussed or implied and not specifically shown, its structure andfunction may also be taken to be similar to the previously disclosed,similarly situated component of FIG. 1 or FIG. 4.

The vehicle brake system 300 may suitably be used on an automotivevehicle having four wheels and a brake for each wheel. One of thedifferences between the brake systems 2 and 300 is that the brake system300 is an electronically controlled brake system for the four wheelswith manual backup for all four of the vehicle brakes 11a, b, c, and d.The brake system 300 includes a primary circuit conduit 302 which is influid communication with the vehicle brakes 11a and 11b through anisolation valve 306. The brake system 300 also includes a secondarycircuit conduit 304 which is in fluid communication with the vehiclebrakes 11c and 11d through an isolation valve 308. The brake system 300further includes a balance valve 310 which selectively isolates thefluid communication between the vehicle brakes 11a and 11b. A balancevalve 312 selectively isolates the fluid communication between thevehicle brakes 11c and 11d. The brake system 300 also includes fourfluid separator units 54a, b, c, and d, positioned between theproportional control valves 5a, b, c, and d and the vehicle brakes 11a,b, c, and d, respectively.

If desired, the brake systems 200 and 300 can further include a suitabledampening circuit 29 and a suitable expansion volume unit 31, eitherseparately or in combination.

FIG. 6 illustrates a specific embodiment of the master cylinder 12 andthe pedal simulator 26 which can be used in the brake systems 2, 200,and 300 of the present invention. The master cylinder 12 is a tandemmaster cylinder, having two service pistons 12a and 12b. The mastercylinder 12 is in fluid communication with the pedal simulator 26 via aconduit, such as the conduit 16. The isolation valve, indicated by theblock 28, is located within the conduit 16 between, and thus in fluidcommunication with the master cylinder 12 and the pedal simulator 26.The optional dampening circuit, indicated by the block 29, is also shownin fluid communication with the pedal simulator via the conduit 16.Although not shown, the expansion volume unit 31 is preferably also influid communication with the pedal simulator 26 through the conduit 16.

FIG. 10 is a schematic view illustration of a vehicle brake system,according to the present invention, which is indicated generally at 350.Many of the components of the brake system 350 are similar to thecomponents disclosed in the brake systems 2, 200, and 300, and functionin a similar manner. Such components will be assigned the same referencenumbers as in the previous brake systems 2, 200, and 300. Where acomponent is shown in FIG. 10 but not specifically discussed, itsfunction and structure may be taken to be similar to that of componentssimilarly situated in the brake systems 2, 200, and 300. Similarly, if acomponent is discussed or implied and not specifically shown, itsexistence and function may also be taken to be similar to the previouslydisclosed similarly situated components.

The vehicle brake system 350 may suitably be used on an automotivevehicle having four wheels and a brake for each wheel. The vehicle brakesystem 350 is comprised of two separate brake systems, a front brakesystem shown generally at 351 352 and a rear brake system showngenerally at 354. The front brake system 351 352 is comprised of twosub-systems: an electrically powered front brake system which includes amotor operated, electronically controlled normal source of pressurizedbrake fluid 4; and a manual supply of pressurized hydraulic brake fluid,embodied as a master cylinder 12. The rear brake system 354 is comprisedof two electronically controlled power cylinders 210 and 212 forsupplying pressurized hydraulic brake fluid to individual wheel brakeunits. The power cylinder 210 uses a linear actuator 214 to drive aspring loaded piston 218 a controlled distance into a cylinder 226. Theoperation of the power cylinder 210 is controlled by the control module.The cylinder 226 is filled with hydraulic brake fluid, which may bepressurized and urged from the cylinder 226 of the power cylinder 210into the brake unit 11d to generate a controlled amount of braking forcewith the vehicle brake 11d. The linear actuator 214 may be any suitabledevice for accurately controlling the position of the piston 218 withrespect to the cylinder 226. A pressure transducer 222 provides a signalto the control module representative of the pressure developed by thepower cylinder 210. Preferably, this pressure signal is used by thecontrol module as a pressure feedback loop for controlling the operationof the power cylinder 210. The control module can modulate the pressureby positioning the linear actuator 214 of the power cylinder 210. Thepower cylinder brake unit 212 is preferably identical in configurationto the power cylinder brake unit 210. The power cylinder 212 uses alinear actuator 216 controlled by the control module to drive a piston220 a controlled distance into a cylinder 228. The cylinder 228 isfilled with hydraulic brake fluid, to selectively effect a controlledamount of braking force in the vehicle brake 11c. The linear actuator216 may be any suitable device for accurately controlling the positionof the piston 220 with respect to the cylinder 228. The pressuretransducer 224 provides a pressure feedback signal to the control modulerepresentative of the pressure developed by the power cylinder 212.Preferably, this pressure signal is used by the control module as apressure feedback loop for controlling the operation of the powercylinder 212. The control module can modulate the pressure bypositioning the linear actuator 216 of the power cylinder 212.

The front brake system 351 352 of the present invention differs from thefront brake units disclosed above, in that there is a singleproportional control valve 51 that controls the hydraulic brake fluidpressure to both front vehicle brakes 11a and 11b. The hydraulic brakefluid is then selectively applied to the vehicle brakes 11a and 11bthrough electrically operated solenoid isolation valves 70a, 70b, 72a,and 72b, as will be described below. Additionally, a pressure isolationvalve 348 is provided which acts to isolate only the accumulator 46 andnot the pump 42. Suitable overpressure protection (not shown) should beprovided for the accumulator 46. The pressure transducer 49 reflects thedischarge pressure of the pump only when the discharge pressure is atleast as high as the pressure in the accumulator 46 when the pressureisolation valve 348 is shut. However the pressure isolation valve 348 isenergized open during normal braking, enabling the pressure transducer49 to reflect the pressure of the hydraulic brake fluid being suppliedto the proportional control valve 51.

Note that two damping circuits 29 are provided, one for each of twopedal simulator 26a and 26b connected, respectively to the conduits 16and 17 out of the master cylinder 12. Only one expansion volume unit 31is shown, in fluid communication with the pedal simulator 26a. Ifdesired, an expansion volume unit 31 could be provided in fluidcommunication with the pedal simulator 26b.

Referring to FIG. 10, the normal source of pressurized hydraulic brakefluid to the front vehicle brakes 11a and 11b is shown generally at 4. Aproportional control valve 51 modulates the pressure of the hydraulicbrake fluid provided to vehicle brakes 11a and 11b according toinstructions received from a control module (not shown). The hydraulicbrake fluid is supplied to the vehicle brakes 11a and 11b throughelectrically operated isolation valves 70a and 70b, respectively. Duringnormal operation, the isolation valves 70a and 70b are de-energized andin the open position, as shown in FIG. 10. The isolation valves 70a and70b thereby allow passage of the hydraulic brake fluid from theproportional control valve 51 to the fluid separator units 54a and 54b.The fluid separator units 54a and 54b function identically to the fluidseparator units in the brake system 2, in that the fluid separator units54a and 54b prevent the hydraulic brake fluids of the normal source 4and the backup source 6 from mixing, thereby preventing a leak from thepiping of the normal source 4 from disabling the backup source 6 ofhydraulic brake fluid, while allowing pressure from the normal source ofpressurized hydraulic brake fluid 4 to be operatively hydraulicallyconnected to the vehicle brakes 11a and 11b. The proportional controlvalve 51 controls the pressure of the hydraulic brake fluid to besupplied to the vehicle brakes 11a and 11b for foundation braking. Inresponse to various driver demands, as signaled by the pressure sensedat the pressure transducers 30 and 32, the proportional control valve 51will be positioned to apply pressurized fluid to the vehicle brakes 11aand 11b, to hold pressure on the brakes 11a and 11b, or to vent pressurefrom the brakes 11a and 11b. Only the one pressure control valve 51 isused to control the pressure for both of the brakes 11a and 11b. Such anarrangement may prove to be less expensive than a separate proportionalcontrol valve 51 for each of the brakes 11a and 11b of the brake system2, for example.

The isolation valves 70a and 70b cooperate with the dump valves 72a and72b to provide digital brake control for antilock braking, vehiclestability control, or traction control functions. For example, atraction control scenario might involve, in a front-wheel drive vehicle,a left front wheel which is losing traction during heavy acceleration.In such a situation, it may be desired to apply the brake 11a, while notapplying the brake 11b. To accomplish this, the isolation valve 70b isshut while the isolation valve 70a remains open. The pressure isolationvalve 348 is opened, and pressurized hydraulic brake fluid from theaccumulator 46 is regulated by the pressure control valve 51 to adesired pressure. The pressurized hydraulic brake fluid is blocked frombeing applied to the brake 11b by the isolation valve 70b, but isallowed, by the open valve 70a to actuate the brake 11a, slowing theindividual wheel until the associated wheel slows and regains traction.The dump valve 72a can then be opened, or the control valve 51 can thenbe deenergized, or both, to bleed pressure from the brake 11a back tothe reservoir 40, as directed by the control module. The pressureisolation valve 348 is then shut, and the pump 42 stopped, if the pump42 was running.

Other control schemes may also be suitably used. For example, if bothfront wheels were slipping under acceleration, but not at the same rate,both the isolation valves 70a and 70b may be shut, the proportionalcontrol valve 51 opened to regulate pressure higher than is to be neededat either wheel, and then the isolation valves 70a and 70b pulsed opento achieve independently controlled pressures needed to slow down therespective wheel. The braking force at each wheel would be controlled bycooperative modulation of the respective isolation valves 70a and 70b,and the dump valves 72a and 72b. In another control arrangement which iscontemplated, the isolation valves 70a and 70b would not be initiallyclosed, but would be closed when the associated wheel began to slow, orwhen the desired brake pressure was reached. In yet another controlscheme which is contemplated, the proportional control valve 51 wouldmodulate pressure in the brakes as needed to achieve the pressure neededfor both of the brakes 11a and 11b, with both the isolation valves 70aand 70b remaining open at all times, and both of the dump valves 72a and72b remaining shut. This would be simple control, but may result in abrake 11a or 11b which did not need as high a brake pressure as theother of the brakes 11a and 11b being braked with greater pressure thanneeded to prevent wheel spin. Therefore it is also contemplated that theproportional control valve 51 would be modulated to control the wheelspin on the wheel operating on the surface with the lower coefficient offriction, while the isolation valves 70a or 70b and the dump valves 72aor 72b would be modulated to control the brake pressure of the otherwheel, on a surface with higher coefficient of friction, at theappropriate lower pressure needed to stop the wheel spin. Thus it isapparent that the arrangement of the brake system 350 provides for greatflexibility in a traction control situation. The same is true in otherbraking situations, such as when antilock braking is required.

As an example, if a need was detected for pulsing the vehicle brakes 11aand b such as would be required to prevent locking up the brakes, or forbraking in slippery road conditions, the isolation valves 70a and 70band the dump valves 72a and 72b could be pulsed open and closed. Thedigital (either on or off) nature of control of the isolation valves 70aand 70b and the dump valves 72a and 72b allows the isolation valves 70aand 70b and the dump valves 72a and 72b to cooperate to rapidlyincrease, decrease, or hold pressure for antilock braking. Othernon-modulated or digital applications of the front brake system 351could be effected as needed with the arrangement of the isolation anddump valves 70a, 70b, 72a, and 72b as shown. Note that it is anticipatedthat the isolation valves 70a and 70b and the dump valves 72a and 72bmay be suitably constructed to provide proportional control of hydraulicbrake fluid passing through the respective valve, thereby permittingfiner control of the hydraulic pressure in the brakes 11a and 11b. Forexample, the isolation valves 70a and 70b may be constructed to enablethe valve to operate in a stable manner when the valve is partiallyopen, allowing a more gradual pressure rise in the associated brake 11aand 11b, which may be desired if the wheel is near lock-up. The dumpvalves 72a and 72b could similarly be constructed to modulate flow ofhydraulic brake fluid therethrough.

As in the previous embodiments of the brake system 2, 200 and 300, uponfailure of the normal source of pressurized hydraulic brake fluid 4 tothe vehicle brakes 11a and 11b, or upon failure of the control module,the backup source 6 of pressurized hydraulic brake fluid supplied by themaster cylinder 12, will be an available source of pressurized hydraulicbrake fluid to be applied to the brakes of the brake system 350,preferably to the front brakes 11a and 11b as illustrated in FIG. 10.The vehicle brakes 11a and 11b supplied by the master cylinder 12 can bedesigned to provide sufficient braking force to safely operate thevehicle with the pressure supplied from the master cylinder 12. Ofcourse, although not illustrated in FIG. 10, it is contemplated that themaster cylinder 12 can be operatively connected to selectively supplypressurized hydraulic brake fluid to the power cylinders 210 and 212, ifdesired. It is also contemplated that separate power supplies may byused to power the motors of the power cylinders 210 and 212 to providean additional level of redundancy and safety to the brake system 350. Ofcourse redundant, independently powered, and cross-checking controlmodules may be utilized to control the operation of the power cylinders210 and 212, and of the proportional control valves 51a and 51b valve51. It is also contemplated that all four of the vehicle brakes 11a, b,c, and d could be supplied from a respective power cylinder similar tothe power cylinder 210. The backup source 6 could be connected to two orfour of the vehicle brakes 11a, b, c, and d. A suitable fluid separatorunit 54a is preferably provided between the power cylinder and theconnection of the backup source 6 in communication with the vehiclebrakes 11a, b, c, and d.

The principle and mode of operation of this invention have beenexplained and illustrated in its preferred embodiment. However, it mustbe understood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

What is claimed is:
 1. A brake system comprising: a normal source ofpressurized hydraulic brake fluid; a backup source of pressurizedhydraulic brake fluid; a vehicle brake which is operated by applicationof pressurized hydraulic brake fluid thereto; a valve for selectivelypreventing the flow of hydraulic brake fluid between the backup sourceand said vehicle brake; a fluid conduit in fluid communication with saidbackup source; a pedal simulator in fluid communication with said backupsource via said fluid conduit, said pedal simulator including a springand a piston acting to compress said spring under the influence ofpressurized hydraulic fluid from said backup source exceeding a firstpressure; and a fluid separator unit for maintaining the integrity ofsaid backup source of pressurized fluid and preventing intermixing ofthe hydraulic brake fluid of said normal source and the hydraulic brakefluid of said backup source and having a movable pressure boundary whichenables, through movement thereof, said normal source of pressurizedhydraulic brake fluid to selectively act upon said vehicle brake via aportion of said backup source when said valve is shut.
 2. The brakesystem of claim 1, further including a brake system brake demanddetection arrangement comprising: a manually operated master cylindercomprising at least a portion of said backup source; asaid fluid conduitbeing in fluid communication with said master cylinder; a pedalsimulator in fluid communication with said master cylinder via saidfluid conduit, said pedal simulator including a spring and a pistonacting to compress said spring under the influence of pressurizedhydraulic fluid from said master cylinder exceeding a first pressure; apressure transducer generating a signal representative of the pressureof said fluid flowing between said master cylinder and said pedalsimulator; and an expansion volume unit in fluid communication with saidmaster cylinder and said pedal simulator via said fluid conduit, saidexpansion volume unit permitting fluid to flow from said master cylinderinto said expansion volume unit when said fluid exceeds a secondpressure less than said first pressure.
 3. The brake system of claim 2wherein said pedal simulator further includes a housing defining a borehaving a first end adapted to be connected in fluid communication withsaid backup source, said bore further having a second end, said pistonbeing slidably disposed in said bore and having a first face and asecond face, said spring engaging said second face of said piston andacting between said piston and a portion of said housing to urge saidfirst face of said piston toward said first end of said bore, and adamping circuit hydraulically interposed between said first end of saidbore and said backup source to present a first cross sectional flow areato fluid flowing from said backup source through said damping circuitinto said housing, and presenting a second cross sectional flow area tofluid flowing from said housing through said damping circuit, the ratioof said second cross sectional flow area to said first cross sectionalflow area being greater than unity.
 4. The brake system of claim 3wherein said ratio is less than 10:1.
 5. The brake system of claim 4wherein said ratio is in the range of 2:1 to 4:1.
 6. The brake system ofclaim 3 further including a relief valve opening above a predeterminedpressure to permit fluid flow through said relief valve from said brakesystem to said housing.
 7. The brake system of claim 6 wherein saidpredetermined pressure is in the range of about 5 bar to about 30 bar.8. The brake system of claim 3 further including a relief valve openingabove a predetermined pressure to permit fluid flow through said reliefvalve from said brake system to said housing.
 9. The brake system ofclaim 8 wherein said predetermined pressure is in the range of about 5bar to about 30 bar.
 10. The brake system of claim 2 wherein said fluidseparator unit has a housing defining a cylinder bore and a pistonslideably disposed therein, said piston having a first working face influid communication with said normal source and a second working face influid communication with said backup source, said first and secondworking faces having substantially similar areas.
 11. The brake systemof claim 2, further including: a brake pedal for operating said mastercylinder; a pedal travel sensor for generating a stroke signalrepresentative of the stroke of said brake pedal; said signal from saidpressure transducer being related to the brake application force appliedby a driver to said brake pedal; and a control unit responsive to ademand signal for controlling said brake system actuator, said demandsignal being generated as a blended function of both said stroke signaland said signal from said pressure transducer wherein, during an initialmovement of said brake pedal, said stroke signal is weighted greaterthan said signal from said pressure transducer, and wherein, during asubsequent movement of said brake pedal, said signal from said pressuretransducer is weighted greater than said stroke signal.
 12. The brakesystem of claim 1 further including a pedal simulator, said pedalsimulator comprising: a housing defining a bore having a first endadapted to be connected in fluid communication with said backup source,said bore further having a second end; asaid piston being slidablydisposed in said bore and having a first face and a second face; asaidspring engaging said second face of said piston and acting between saidpiston and a portion of said housing to urge said first face of saidpiston toward said first end of said bore; and a damping circuithydraulically interposed between said first end of said bore and saidbackup source to present a first cross sectional flow area to fluidflowing from said backup source through said damping circuit into saidhousing, and presenting a second cross sectional flow area to fluidflowing from said housing through said damping circuit, the ratio ofsaid second cross sectional flow area to said first cross sectional flowarea being greater than unity.
 13. The brake system of claim 12 whereinsaid ratio is less than 10:1.
 14. The brake system of claim 13 whereinsaid ratio is in the range of 2:1 to 4:1.
 15. The brake system of claim12 further including a relief valve opening above a predeterminedpressure to permit fluid flow through said relief valve from said brakesystem to said housing.
 16. The brake system of claim 15 wherein saidpredetermined pressure is in the range of about 5 bar to about 30 bar.17. The brake system of claim 1 wherein said fluid separator unit has ahousing defining a cylinder bore and a, said piston being slideablydisposed therein, said piston having a first working face in fluidcommunication with said normal source and a second working face in fluidcommunication with said backup source, said first and second workingfaces having substantially similar areas.
 18. A brake system comprising:a brake pedal for operating a brake system actuator; a pedal travelsensor for generating a stroke signal representative of the stroke ofsaid brake pedal; a brake system sensor for generating a force signalrepresentative of the brake application force applied by a driver tosaid brake pedal; a control unit responsive to a demand signal forcontrolling said brake system actuator, said demand signal beinggenerated as a blended function of both said stroke, signal and saidforce signal wherein, during a first part of the stroke of said brakepedal, said stroke signal is weighted greater than said force signal,and wherein, during a second part of the stoke of said brake pedal, saidforce signal is weighted greater than said stroke signal.
 19. Anelectro-hydraulic brake system comprising: a reservoir of hydraulicbrake fluid; a pump having a suction port and a discharge port, saidsuction port being connected in fluid communication with said reservoir;a first fluid conduit being connected in fluid communication with saiddischarge port of said pump; a fluid separator unit having a housingwith a bore defined therethrough, said bore having a first end and asecond end, said first end of said bore being connected in fluidcommunication with said discharge port of said pump via said first fluidconduit, said fluid separator unit further including a piston slidinglydisposed in said bore and a spring disposed to urge said piston towardsaid first end of said bore; a second fluid conduit connected in fluidcommunication with said second end of said fluid separator unit; avehicle brake connected in fluid communication with said second end ofsaid fluid separator unit via said second fluid conduit; a third fluidconduit connected in fluid communication with said vehicle brake; ahydraulic master cylinder connected in fluid communication with saidvehicle brake via said third fluid conduit; an electrically-operatedvalve disposed in said third fluid conduit, said valve preventing theflow of hydraulic brake fluid between said master cylinder and saidvehicle brake when closed, said valve being open to permit the flow ofhydraulic brake fluid between said master cylinder and said vehiclebrake when said valve is electrically deenergized; a fourth fluidconduit connected in fluid communication with said master cylinder andsaid third fluid conduit; a pedal simulator connected in fluidcommunication with said master cylinder via said fourth fluid conduit;an second electrically-operated valve disposed in said fourth fluidconduit, said second valve being closed to prevent the flow of hydraulicbrake fluid between said master cylinder and said pedal simulator whensaid second valve is deenergized, said second valve permitting the flowof hydraulic brake fluid between said master cylinder and said pedalsimulator when said second valve is open; and a damping circuithydraulically interposed between said master cylinder and said pedalsimulator, said damping circuit comprising, in parallel flow paths, anorifice and a check valve such that said damping circuit presents afirst cross sectional flow area to fluid flowing from said mastercylinder through said damping circuit into said pedal simulator, andpresenting a second cross sectional flow area, different from said firstcross sectional flow area, to fluid flowing from said pedal simulator tosaid master cylinder through said damping circuit.
 20. Theelectro-hydraulic brake system of claim 19 further including a thirdelectrically-operated valve disposed in said first fluid conduit, saidthird valve preventing fluid communication between said pump and saidfluid separator unit when said third valve is closed, said third valvepermitting fluid communication between said pump and said fluidseparator unit when said third valve is open, the electro-hydraulicbrake system further including fifth fluid conduit having a first endconnected in fluid communication with said first fluid conduit and saidfluid separator unit and having a second connected in fluidcommunication with said reservoir, the electro-hydraulic brake systemfurther including a fourth electrically-operated valve disposed in saidfifth fluid conduit, said fourth valve preventing fluid communicationbetween said fluid separator unit and said reservoir when said fourthvalve is closed, said fourth valve permitting fluid communicationbetween said fluid separator unit and said reservoir when said fourthvalve is open.
 21. A hydraulic brake system for a vehicle comprising:wheel brakes for four wheels, in which the wheels are distributed with afirst and a second wheel brake on a first vehicle axle and a third and afourth wheel brake on a second vehicle axle; a normal hydraulic energysource, having electrically controllable brake valve devices disposedbetween said energy source and said wheel brakes; a brake pedal; asensor generating a first signal indicative of the position of saidbrake pedal; a second sensor generating a second signal indicative ofthe force exerted by a driver on said brake pedal; a master cylindersupplying two brake circuits, said master cylinder being actuated bysaid brake pedal and being intended for carrying out a backup brakeoperation by muscle-powered energy via said brake pedal, each brakecircuit being in fluid communication with a respective one of said firstand second wheel brakes; a respective normally open isolation valvebeing disposed between said master cylinder and said wheel brakes ineach of said two brake circuits, each of said isolation valves beingswitched into a closed position when said wheel brakes are supplied withfluid from said normal hydraulic energy source; a respective fluidseparator unit being interposed between each of said first and secondwheel brakes of said first vehicle axle and an associated one of theelectrically controllable brake valve devices, said fluid separatorunits having movable components forming a pressure boundary that enablessaid normal source to selectively act upon said vehicle brake via aportion of said backup source, said first and second wheel brakes beingconnected to a respective one of said isolation valves associated withsaid two brake circuits of said master cylinder; and a control unit forcontrolling said normal hydraulic energy source and said isolationvalves, said control unit responding as a blended function of both saidfirst signal and said second signal, with the contribution of the secondsignal relative to the first signal generally varying as a function ofthe first signal.
 22. The hydraulic brake system of claim 18, furthercomprising: wheel brakes for two wheels, in which the wheels aredistributed at each end of a front vehicle axle; a normal source ofpressurized hydraulic brake fluid, having electrically controllablebrake valve devices disposed between said normal source and said wheelbrakes, a master cylinder comprising at least a portion of said brakesystem actuator and supplying two brake circuits, said master cylinderbeing actuated by said brake pedal and being intended for carrying out abackup brake operation by muscle-powered energy via said brake pedal,each of said brake circuits being in fluid communication with arespective one of said wheel brakes; and a respective normally openisolation valve being disposed between said master cylinder and saidrespective one of said wheel brakes in each brake circuit, each of saidisolation valves being electrically switched into a closed position whensaid wheel brakes are supplied with fluid from said normal source, andwherein at least the electrically controllable brake valve devices arecontrolled by said control unit.
 23. The hydraulic brake system of claim22, said normal source including a motor driven pump for pumpinghydraulic brake fluid from a reservoir, wherein said electricallycontrollable brake valve devices are arranged to block a respective flowpath from said normal source to said wheel brakes and to open arespective flow path from said wheel brakes to said reservoir when nobraking is being demanded.
 24. The hydraulic brake system of claim 22,said normal source including a motor driven pump for pumping hydraulicbrake fluid from a reservoir, wherein said electrically controllablebrake valve devices are arranged to block a respective flow path fromsaid normal source to said wheel brakes and to open a respective flowpath from said wheel brakes to said reservoir when no braking is beingdemanded.
 25. The brake system of claim 18, further comprising: an axleof a vehicle; a first wheel brake mounted on said axle; a second wheelbrake mounted on said axle; a normal source of pressurized hydraulicbrake fluid adapted to selectively supply hydraulic brake fluid to saidfirst wheel brake and said second wheel brake; a backup source ofpressurized hydraulic brake fluid comprising a master cylinder; a firstbackup fluid conduit extending between said master cylinder and saidfirst wheel brake to selectively provide fluid communication betweensaid backup source and said first wheel brake; and a second backup fluidconduit extending between said master cylinder and said second wheelbrake to selectively provide fluid communication between said backupsource and said second wheel brake.
 26. The hydraulic brake system ofclaim 25, wherein said normal source is under the control of saidcontrol unit.
 27. The brake system of claim 18, further comprising:wheel brakes for two wheels, in which the wheels are distributed at eachend of a front vehicle axle; a normal source of pressurized hydraulicbrake fluid, having electrically controllable brake valve devicesdisposed between said normal source and said wheel brakes, saidelectrically controllable brake valve devices being controlled by acontrol unit in response to a braking demand signal; a master cylindersupplying two brake circuits, said master cylinder being actuated bysaid brake pedal and being intended for carrying out a backup brakeoperation by muscle-powered energy via said brake pedal, each of saidbrake circuits being in fluid communication with a respective one ofsaid wheel brakes; and a respective normally open isolation valve beingdisposed between said master cylinder and said respective one of saidwheel brakes in each brake circuit, each of said isolation valves beingelectrically switched into a closed position when said wheel brakes aresupplied with fluid from said normal source.
 28. The hydraulic brakesystem of claim 27, said normal source including a motor driven pump forpumping hydraulic brake fluid from a reservoir, wherein saidelectrically controllable brake valve devices are arranged to block arespective flow path from said normal source to said wheel brakes and toopen a respective flow path from said wheel brakes to said reservoirwhen no braking is being demanded.
 29. The hydraulic brake system ofclaim 18, further comprising: wheel brakes for two wheels, in which thewheels are distributed at each end of a front vehicle axle; a hydraulicfluid reservoir; a normal source of pressurized hydraulic brake fluid,having a motor-driven pump for pumping hydraulic brake fluid from saidreservoir; a master cylinder comprising at least a portion of said brakesystem actuator and supplying two brake circuits, said master cylinderbeing actuated by said brake pedal and being intended for carrying out abackup brake operation by muscle-powered energy via said brake pedal,each of said brake circuits being in fluid communication with arespective one of said wheel brakes; and a respective electricallycontrollable brake valve device associated with each of said wheelbrakes, said electrically controllable brake valve devices beingarranged to block a respective flow path from said normal source to saidwheel brakes and to open a respective flow path from said wheel brakesto said reservoir when no braking is being demanded.
 30. The brakesystem of claim 18, further comprising: wheel brakes for four wheels, inwhich the wheels are distributed with a first and second wheel brake ona first vehicle axle and a third and a fourth wheel brake on a secondvehicle axle; a normal hydraulic energy source, having electricallycontrollable brake valve devices disposed between said energy source andsaid wheel brakes; said brake system sensor actuated by said brakepedal, for carrying out brake operations by operation of theelectrically controllable brake valve devices; a master cylindersupplying two brake circuits, said master cylinder being actuated bysaid brake pedal and being intended for carrying out a backup brakeoperation by muscle-powered energy via said brake pedal, each brakecircuit being in fluid communication with at least one of said wheelbrakes; a respective normally open isolation valve being disposedbetween said master cylinder and said wheel brakes in each of said twobrake circuits, each of said isolation valves being switched into aclosed position when said wheel brakes are supplied with fluid from saidnormal hydraulic energy source, and wherein at least the electricallycontrollable brake valve devices are controlled by a control unit; and arespective fluid separator unit being interposed between each of saidfirst and second wheel brakes of said first vehicle axle and anassociated one of the electrically controllable brake valve devices,said first and second wheel brakes being connected to a respective oneof said isolation valves associated with said two brake circuits of saidmaster cylinder.
 31. The brake system of claim 18, further comprising:wheel brakes for two wheels, in which the wheels are distributed at eachend of a front vehicle axle; a hydraulic fluid reservoir; a normalsource of pressurized hydraulic brake fluid, having a motor-driven pumpfor pumping hydraulic brake fluid from said reservoir; a master cylindersupplying two brake circuits, said master cylinder being actuated bysaid brake pedal and being intended for carrying out a backup brakeoperation by muscle-powered energy via said brake pedal, each of saidbrake circuits being in fluid communication with a respective one ofsaid wheel brakes; and a respective electrically controllable brakevalve device associated with each of said wheel brakes, saidelectrically controllable brake valve devices being arranged to block arespective flow path from said normal source to said wheel brakes and toopen a respective flow path from said wheel brakes to said reservoirwhen no braking is being demanded.
 32. The brake system of claim 1,further comprising: a second vehicle brake, each of said vehicle brakeand said second vehicle brake comprising respective wheel brakes for twowheels, in which the wheels are distributed at each end of a frontvehicle axle; electrically controllable brake valve devices disposedbetween said normal source and said wheel brakes, said electricallycontrollable brake valve devices being controlled by a control unit inresponse to a braking demand signal; a brake pedal; said backup sourcecomprising a master cylinder supplying two brake circuits, said mastercylinder being actuated by said brake pedal and being intended forcarrying out a backup brake operation by muscle-powered energy via saidbrake pedal, each of said brake circuits being in fluid communicationwith a respective one of said wheel brakes; and a respective normallyopen isolation valve being disposed between said master cylinder andsaid respective one of said wheel brakes in each brake circuit, each ofsaid isolation valves being electrically switched into a closed positionwhen said wheel brakes are supplied with fluid from said normal source,one of said normally open isolation valves comprising said valve forselectively preventing the flow of hydraulic brake fluid between thebackup source and said vehicle brake.
 33. The brake system of claim 32,said normal source including a motor driven pump for pumping hydraulicbrake fluid from a reservoir, wherein said electrically controllablebrake valve devices are arranged to block a respective flow path fromsaid normal source to said wheel brakes and to open a respective flowpath from said wheel brakes to said reservoir when no braking is beingdemanded.
 34. The brake system of claim 1, further comprising: a secondvehicle brake, said vehicle brake and said second vehicle brake beingmounted on an axle of a vehicle, said normal source of pressurizedhydraulic brake fluid adapted to selectively supply hydraulic brakefluid to said vehicle brake and said second vehicle brake, said backupsource of pressurized hydraulic brake fluid comprising a mastercylinder; a first backup fluid conduit extending between said mastercylinder and said first vehicle brake to selectively provide fluidcommunication between said backup source and said first vehicle brake;and a second backup fluid conduit extending between said master cylinderand said second vehicle brake to selectively provide fluid communicationbetween said backup source and said second vehicle brake.
 35. The brakesystem of claim 1, further comprising: a second vehicle brake, saidvehicle brake and said second vehicle brake distributed on a firstvehicle axle; a third and a fourth vehicle brake on a second vehicleaxle; electrically controllable brake valve devices disposed betweensaid normal source of pressurized hydraulic brake fluid and said vehiclebrakes; a brake pedal; a first brake system sensor that is actuated bysaid brake pedal, for carrying out brake operations by operation of theelectrically controllable brake valve devices; a master cylindersupplying two brake circuits, said master cylinder being actuated bysaid brake pedal and being intended for carrying out a backup brakeoperation by muscle-powered energy via said brake pedal, each brakecircuit being in fluid communication with at least one of said vehiclebrakes; a respective normally open isolation valve being disposedbetween said master cylinder and said vehicle brakes in each of said twobrake circuits, each of said isolation valves being switched into aclosed position when said vehicle brakes are supplied with fluid fromsaid normal hydraulic energy source, and wherein at least theelectrically controllable brake valve devices are controlled by acontrol unit; and a respective one of said fluid separator unit and asecond fluid separator unit being interposed between each of said firstand second vehicle brakes of said first vehicle axle and an associatedone of the electrically controllable brake valve devices, said first andsecond vehicle brakes being connected to a respective one of saidisolation valves associated with said two brake circuits of said mastercylinder.
 36. The brake system of claim 1, further comprising: a secondvehicle brake, each of said vehicle brake and said second vehicle brakecomprising respective wheel brakes for two wheels, in which the wheelsare distributed at each end of a front vehicle axle; a hydraulic fluidreservoir; said normal source of pressurized hydraulic brake fluidhaving a motor-driven pump for pumping hydraulic brake fluid from saidreservoir; a brake pedal; said backup source of pressurized hydraulicfluid comprising a master cylinder supplying two brake circuits, saidmaster cylinder being actuated by said brake pedal and being intendedfor carrying out a backup brake operation by muscle-powered energy viasaid brake pedal, each of said brake circuits being in fluidcommunication with a respective one of said wheel brakes; and arespective electrically controllable brake valve device associated witheach of said wheel brakes, said electrically controllable brake valvedevices being arranged to block a respective flow path from said normalsource to said wheel brakes and to open a respective flow path from saidwheel brakes to said reservoir when no braking is being demanded.